Tetrahedral Rotation Research Articles

Variable-Temperature 17O NMR Study of Oxygen Motion in the Anionic Conductor Bi26Mo10O69

Variable-temperature O-17 NMR spectroscopy, spanning a temperature range from -238 to 1000 degrees C, has been used to investigate mechanisms for ionic conduction in Bi26Mo10O69, a material that contains both MoO42- tetrahedra and [Bi12O14](infinity)(8-) columns. Two O-17 NMR resonances are observed that are assigned to oxygen atoms in the MoO42- tetrahedra and in the [Bi12O14](infinity)(8-) columns. On the basis of the nutation curves for the two groups of resonances, extremely rapid, but local, reorientational motion of the MoO42- units occurs at -70 degrees C and above (with a frequency of >50 kHz), whereas the Bi-O oxygen ions are rigid in this temperature regime. This is confirmed by both an analysis of the line broadening of the MoO42- satellite transitions (under MAS) and the spin-lattice relaxation (T-1) times of these sites, the T-1 times indicating that the MoO42- reorientation rates rapidly increase, reaching > 100 MHz at 400 degrees C. Line narrowing of the MoO42- central-transition resonance indicates that exchange between the tetrahedral units, a motion required for long-range anionic conduction, is much slower, involving only jump rates of approximately 1 kHz at 200 degrees C. Both the changes in line width of the MoO42- resonance, and the jump in the T-1 times of the oxygen atoms in the [Bi12O14](infinity)(8-) columns at around the triclinic-monoclinic phase transition temperature (310 degrees C) are consistent with a mechanism for motion involving all the oxygen atoms. The predicted conductivity based on the [Bi-O] T-1 times is now of the order of that extracted from ac impedance measurements reported by Vannier et al. (J. Solid State Chem. 1996, 122, 394). On the basis of this detailed NMR analysis, we propose that motion at ambient temperatures primarily involves the MoO42- tetrahedral rotation: exchange between these sites is very slow. At higher temperatures (above 310 degrees C) the conduction process now appears to involve the oxygen atoms coordinated to Bi3+, in the [Bi12O14](infinity)(8-) columns, and most likely in the partially vacant O[19] site. The involvement of these sites allows for long-range conduction processes that do not involve concerted, multiple Mo-O bond breakages.

Thermal expansion and dehydroxylation of phengite micas

Phengite samples (2M 1 and 3T politypes) and a synthetic end-member muscovite specimen were studied by in situ high-temperature synchrotron radiation X-ray diffraction. The measured volume thermal expansion of 2M 1 phengite (<α V> ≈ 36.6 × 10−6 K−1) was systematically greater than <α V> of the 3T polytype (≈33.3 × 10−6 K−1). A positive linear correlation between the average thermal expansion on (001) plane and the mean tetrahedral rotation angle at ambient condition is proposed on the ground of new measurements and literature data. Dehydroxylation processes were observed in 2M 1, starting at 1,000 K in 3T at 800 and 945 K in synthetic muscovite. Rietveld refinements allowed a determination of structural variations upon heating of phengite samples and their dehydroxylate phases. The phengite structure expands by regularizing the tetrahedral sheet and by reducing the bond length differences between the outer and inner coordination shell of the interlayer site. The dehydroxylate phase derived from 2M 1 is characterized by fivefold polyhedra in the low temperature form as a consequence of two OH groups reacting to form H2O + O (residual). The dehydroxylate exhibits an increase of the cation–cation distances along the M–Or–M bonds with respect to low-temperature phengite structures. For the 3T phase, we were unable to achieve completion of dehydroxylation. The refined structural model of the dehydroxylate phase shows two hydroxyl sites, but at a short distance from one another. This result suggests that the dehydroxylation reaction did not proceed to completion.

Crystal structure of britvinite [Pb7(OH)3F(BO3)2(CO3)][Mg4.5(OH)3(Si5O14)]: A new layered silicate with an original type of silicon-oxygen networks

The crystal structure of a new mineral britvinite Pb7.1Mg4.5(Si4.8Al0.2O14)(BO3)(CO3)[(BO3)0.7(SiO4)0.3](OH, F)6.7 from the Langban iron-manganese skarn deposit (Varmland, Sweden) is determined at T = 173 K using X-ray diffraction (Stoe IPDS diffractometer, λMoKα, graphite monochromator, 2θmax = 58.43°, R = 0.052 for 6262 reflections). The main crystal data are as follows: a = 9.3409(8) A, b = 9.3579(7) A, c = 18.8333(14) A, α = 80.365(6)°, β = 75.816(6)°, γ = 59.870(5)°, V = 1378.7(2) A3, space group P1, Z = 2, and ρcalcd = 5.42 g/cm3. The idealized structural formula of the mineral is represented as [Pb7(OH)3F(BO3)2(CO3)][Mg4.5(OH)3(Si5O14)]. It is demonstrated that the mineral britvinite is a new representative of the group of mica-like layered silicates with structures in which three-layer (2: 1) “sandwiches” composed of tetrahedra and octahedra alternate with blocks of other compositions, such as oxide, oxide-carbonate, oxide-carbonate-sulfate, and other blocks. The tetrahedral networks (Si5O14)∞∞ consisting of twelve-membered rings are fragments of the britvinite structure. Similar networks also form crystal structures of the mineral zeophyllite and the synthetic phase Rb6Si10O23. In the crystal structures under consideration, the tetrahedral networks differ in the rotation of tetrahedra with respect to the layer plane.

Single-crystal X-ray studies of trioctahedral micas coexisting with dioctahedral micas in metamorphic sequences from western Maine

A crystal-chemical study of thirteen biotite (twelve of 1 M polytype and one of 2 M 1 polytype) and four muscovite samples was made. The biotite coexists with the muscovite. Samples are from metamorphic terranes and from granitic and granodioritic bodies occurring in three areas of western Maine. The metamorphic mineral zones identified by mineral compatibilities are, in order of increasing metamorphic grade: the Lower Sillimanite Zone (LSZ), the Upper Sillimanite Zone (USZ), and the K-feldspar + Sillimanite Zone (K + SZ). The muscovite compositions cluster near ideal muscovite and display a small celadonite substitution and a small, but variable, paragonite substitution. The biotite composition has values of VIMg2+/VI(Mg2+ + Fe2+) ranging from 0.26 to 0.54 and significant octahedral Al content (0.48 ≤ VIAl ≤ 0.72 apfu in metamorphic biotite samples, 0.51 ≤ VIAl ≤ 0.67 in those from granites). In trioctahedral micas from western Maine and especially in those with graphite, there are a greater number of interlayer vacancies than in common micas. Interlayer vacancies have an increase in interlayer cation-basal oxygen atom distances and a decrease in tetrahedral flattening angle τ, thus suggesting a reduced interlayer charge. With a few exceptions, tetrahedral rotation angle α is related to crystallization temperature. In particular, α decreases with a temperature increase, and α is also related to octahedral chemical substitutions. Results tentatively suggest, for micas from metamorphic environments, a direct influence of genetic parameters ( T and f O2) on mica crystal structure, and not just chemical composition.

Compressibility trends of the clinopyroxenes, and in-situ high-pressure single-crystal X-ray diffraction study of jadeite

The crystal structure of a natural jadeite, NaAlSi2O6, was studied at room temperature over the pressure range 0–9.17 GPa using single-crystal X-ray diffraction. Unit-cell data were determined at 16 pressures, and intensity data were collected at nine of these pressures. A third-order Birch-Murnaghan equation of state fit to the P-V data yielded V = 402.03(2) A3, K = 136.5(14) GPa, and K ′ = 3.4(4) Jadeite exhibits strongly anisotropic compression with unit strain axial ratios of 1.00:1.63:2.10. Silicate chains become more O-rotated with pressure, reducing ∠O3-O3-O3 from 174.7(1)° at ambient pressure to 169.2(6)° at 9.17 GPa and bringing the anions of jadeite closer to a cubic closest-packed arrangement. No evidence of a phase transition was observed over the studied pressure range. In an effort to understand pyroxene compressibilities, selected clinopyroxene bulk moduli were plotted against ambient unit-cell volumes. Two trends were identified and are explained in terms of differences in M2-O3 bonding topologies and the geometric relationship of the bonds with tetrahedral rotation in the silicate chains. Bonds positioned to favor the tetrahedral rotation upon compression are termed “sympathetic,” whereas bonds positioned to resist the rotation are termed “antipathetic.” Examination of the different pyroxene structures indicates that structures containing antipathetic M2-O3 bonds are less compressible than those with only sympathetic M2-O3 bonds. This behavior has not been previously recognized.

Triclinic apatites

Apatites commonly adopt P6(3)/m hexagonal symmetry. More rarely, monoclinic chemical analogues have been recognized, including the biologically significant hydroxyapatite, Ca(10)(PO(4))(6)(OH)(2), but the driving force towards lower symmetry has not been systematically examined. A combination of diffraction observations and ab initio calculations for Ca(10)(AsO(4))(6)F(2) and Ca(10)(VO(4))(6)F(2) show these materials are triclinic P\bar 1 apatites in which the AsO(4) and VO(4) tetrahedra tilt to relieve stress at the metal and metalloid sites to yield reasonable bond-valence sums. An analysis of the triclinic non-stoichiometric apatites La(10 - x)(GeO(4))(6)O(3 - 1.5x) and Ca(10)(PO(4))(6)(OH)(2 - x)O(x/2) confirms this scheme of tetrahedral rotations, while Cd(10)(PO(4))(6)F(2) and Ca(10)(CrO(4))(6)F(2) are predicted to be isostructural. These distortions are in contrast to the better known P112(1)/b monoclinic dimorphs of chloroapatite and hydroxyapatite, where the impetus for symmetry reduction is ordered anion (OH(-) and Cl(-)) displacements which are necessary to obtain acceptable bond lengths. These results are important for designing apatites with specific structural and crystal-chemical characteristics.

Behavior of 10-Å phase at low temperatures

The thermal evolution of 10-A phase Mg3Si4O10(OH)2·H2O, a phyllosilicate which may have an important role in the storage/release of water in subducting slabs, was studied by X-ray single-crystal diffraction in the temperature range 116–293 K. The lattice parameters were measured at several intervals both on cooling and heating. The structural model was refined with intensity data collected at 116 K and compared to the model refined at room temperature. As expected for a layer silicate on cooling in this temperature range, the a and b lattice parameters undergo a small linear decrease, α a = 1.7(4) 10−6 K−1 and α b = 1.9(4) 10−6 K−1, where α is the linear thermal expansion coefficient. The greater variation is along the c axis and can be modeled with the second order polynomial c T = c 293(1 + 6.7(4)10−5 K−1ΔT + 9.5(2.5)10−8 K−2(ΔT)2) where ΔT = T − 293 K; the monoclinic angle β slightly increased. The cell volume thermal expansion can be modeled with the polynomial V T = V 293 (1 + 8.0 10−5 K−1 ΔT + 1.4 10−7 K−2 (ΔT)2) where ΔT = T − 293 is in K and V in A3. These variations were similar to those expected for a pressure increase, indicating that T and P effects are approximately inverse. The least-squares refinement with intensity data measured at 116 K shows that the volume of the SiO4 tetrahedra does not change significantly, whereas the volume of the Mg octahedra slightly decreases. To adjust for the increased misfit between the tetrahedral and octahedral sheets, the tetrahedral rotation angle α changes from 0.58° to 1.38°, increasing the ditrigonalization of the silicate sheet. This deformation has implications on the H-bonds between the water molecule and the basal oxygen atoms. Furthermore, the highly anisotropic thermal ellipsoid of the H2O oxygen indicates positional disorder, similar to the disorder observed at room temperature. The low-temperature results support the hypothesis that the disorder is static. It can be modeled with a splitting of the interlayer oxygen site with a statistical distribution of the H2O molecules into two positions, 0.6 A apart. The resulting shortest Obas–OW distances are 2.97 A, with a significant shortening with respect to the value at room temperature. The low-temperature behavior of the H-bond system is consistent with that hypothesized at high pressure on the basis of the Raman spectra evolution with P.

The local crystal chemistry and dielectric properties of the cubic pyrochlore phase in the Bi 2O 3[sbnd]M2+O [sbnd]Nb 2O 5 ( M2+=Ni 2+ and Mg 2+) systems

The composition, dielectric properties and inherent displacive disorder of a Bi-based, misplaced-displacive cubic pyrochlore phase found in two ternary Bi 2O 3 M 2+O Nb 2O 5 ( M=Ni and Mg) systems has been investigated. The dielectric permittivities (up to 1 MHz) of (Bi 0.825Ni 0.125□ 0.05) 2(Ni 0.25Nb 0.75) 2O 7 and (Bi 0.835Mg 0.085□ 0.08) 2(Mg 0.235Nb 0.765) 2O 7 at room temperature are found to be 116 and 151, respectively, while the dielectric loss tangents are 0.00065 and 0.00042, respectively, at 100 kHz. A highly structured characteristic diffuse intensity distribution apparent in electron diffraction is reported in both cases and partially interpreted in terms of large amplitude, β-cristobalite-type tetrahedral rotations of the O’ A 2 tetrahedral framework sub-structure of the ideal pyrochlore structure type. Bond valence sum calculations are used to investigate the local crystal chemistry responsible for this displacive disorder.

Local crystal chemistry, structured diffuse scattering and the dielectric properties of (Bi 1−xY x) 2( MIIINb V)O 7 ( M=Fe 3+, In 3+) Bi-pyrochlores

Electron diffraction is used to investigate the large amplitude displacive disorder characteristic of the Bi 2( M IIINb V)O 7 Bi-pyrochlores, Bi 2InNbO 7 and Bi 2FeNbO 7, as well as of their A site substituted Bi 1.5Y 0.5InNbO 7 and Bi 1.5Y 0.5FeNbO 7 variants. Highly structured diffuse distributions in the form of {110}* sheets of diffuse intensity perpendicular to the six 〈110〉 directions of real space along with 〈111〉* rods of diffuse intensity perpendicular to the four {111} real space planes are observed. The existence of this structured diffuse scattering is interpreted in terms of large amplitude, β-cristobalite-type tetrahedral rotations of the O′ A 2 tetrahedral framework sub-structure of the ideal pyrochlore structure type. Bond valence sum calculations are used to understand the local crystal chemistry responsible for such displacive disorder. The frequency-dependent dielectric properties of Bi 2InNbO 7 and Bi 2FeNbO 7 are also investigated along with the effect upon them of A site doping with Y.

Upper limit of the tetrahedral rotation angle and factors affecting octahedral flattening in synthetic and natural 1M polytype C2/m space group micas

We have used recently developed quantitative crystal chemical models and a simple structural free-energy model to examine and interpret: (1) previously reported powder X-ray diffraction data for several trioctahedral mica solid solution series (64 synthetic powder samples between the Mg, Co, Ni, and Fe end-members, with different degrees of oxidation, vacancy contents, and Al/Si ratios; indexed as 1M polytype, space group C2/m; supplemented here by 57 Fe Mössbauer spectroscopy to obtain accurate iron-site populations of IV Fe 3+ , VI Fe 3+ , and VI Fe 2+ ), and (2) 175 previously published single-crystal refinements comprising 138 natural and 37 synthetic 1M mica samples refined in space group C2/m. The crystal chemical models were validated by comparisons between predicted and measured relations between structural parameters, and needed model parameters and their uncertainties were extracted, using the single-crystal refinements. Two main results arise. First, an observed limit value of the b lattice parameter in certain synthetic solid solution series is shown to correspond to an upper limit value for the tetrahedral rotation angle α of αmax = 9.5° for AlSi 3 tetrahedral sheets in K-rich micas. This upper limit is also clearly seen in the single-crystal refinement data for those Krich single-crystals that have near-AlSi 3 tetrahedral compositions. We argue that the (tetrahedral sheet composition dependent) upper limit of tetrahedral rotation is an intrinsic property of the tetrahedral sheet (presumably corresponding to an intra-tetrahedral-sheet bond-bending limit) rather than arising either from interactions with the interlayer cations or from an octahedral sheet lateral-contraction limit. Second, we find that, except in the extreme cases where one approaches the lower (α = 0°) or upper (α = α max ) tetrahedral rotation limits, the magnitude of the octahedral flattening angle ψ is predominantly determined by octahedral cation stereo-chemical bonding requirements (and other intra-octahedral-sheet properties such as intra-sheet bond bending and intra-sheet electrostatic forces) rather than arising from tetrahedral-octahedral inter-sheet interactions (as generally argued or assumed). In addition, we corroborate a previously reported difference in the crystal chemical behaviors of trivalent octahedral cation (Fe 3+ , Al 3+ ) and vacancy-bearing trioctahedral micas relative to samples that contain only divalent octahedral cations (e.g., Fe-Mg, Fe-Ni, Mg-Ni, and Co-Mg synthetic series); their b vs. average octahedral metal-oxygen bond-length behaviors are dramatically different, a result that is consistent with our proposed dominant stereo-chemical control of ψ.

Potassium coordination in trioctahedral micas investigated by K-edge XANES spectroscopy

Fine-grained homogeneous powder samples of thirteen trioctahedral micas, mostly intermediate members of the phlogopite – annite solid solution series, and samples close to the phlogopite, fluor-phlogopite and tetra-ferriphlogopite end members have been examined at the potassium K-edge by X-ray absorption fine structure spectroscopy. The interlayer K+ cation is in a coordination that is certainly lower than 12, in contrast to what is expected from the ideal hexagonal symmetry model of the mica structure, and approaches – but it does not reach – coordination 6, as it should be when the effective ligands are the three nearest outer bridging oxygens of two facing upper and lower tetrahedral sheets. The observed range of coordinations implies that only some of the three inner bridging oxygen atoms in each sheet are involved, thus leading to 6±(1 … 6) effective configurations depending on the composition of the individual mica terms. The effective coordination number was found to vary continuously with composition from 11 to 7 and to be related to the tetrahedral rotation angle (α) according to two different linear relationships for the phlogopite – annite series (Fe2+Mg−1 exchange vector, involving the octahedral sheet only) and the phlogopite – tetra-ferriphlogopite series (Fe3+Al−1 vector, involving the tetrahedral sheet), respectively. Substitutions affecting either the A cation in the interlayer or the X anion in the octahedral sheet also affect the observed trends. In particular, the latter substitution effect is best seen in two near end member phlogopites, where the fluorine to hydroxyl substitution (F− (OH)−−1 exchange vector), which greatly changes the α tetrahedral rotation angle is, reflected in the experimental K XANES spectra by modifying not only the energy but also the intensities of most multiple scattering features.

The 2M1 dioctahedral mica polytype: A crystal chemical study

AbstractThe structure of dioctahedral true micas such as muscovite and celadonitic muscovite (2M1 polytype, space group C2/c) is mostly affected by variations of the octahedral Al (VIAl) content. Crystals with greater Mg, Fe substitutions (i.e. celadonitic muscovite) reduce the dimensional difference between the larger trans-oriented M1 site and smaller cis-oriented M2 octahedral site. The octahedral anionic position O4 is displaced from the center of the hexagon, defined by 031 and 032 oxygen atoms (i.e. ‘octahedral hexagon’), both on and off the (001) plane. The distance between interlayer cation A and O4 is smaller in more substituted species, thus providing different orientations of the O4−H vector, as a function of VIAl. Octahedral distances (&lt;M2−O3&gt; and &lt;M2−O4&gt; are expressed as a function of cell parameters and VIA1 content, thus allowing an approximate estimate of site dimensions. These approximations are useful when a detailed structural refinement is not available. In celadonitic muscovite, the octahedral hexagon mean edge (&lt;O31−O32&gt;Hex) is not significantly affected by VIA1 content. The VIA1 increase produces both a decrease in cell lateral dimensions and a distorted ‘octahedral hexagon’. The decrease in a and b is consistent with a decrease of &lt;O31−O32&gt;Hex, whereas the distortion of the’ octahedral hexagon’ is consistent with an increase of (&lt;031–032&gt;Hex), because an irregular hexagon produces a longer mean edge than a regular hexagon of equal area.The tetrahedral mean basal edge (VI&lt;O−O&gt;bassal) is reduced as celadonitic substitution progresses. The tetrahedral rotation angle, α was thus found to increase from celadonite to muscovite. However, in muscovite with VIAl content between 1.8 and 2.0 atoms per formula unit (a.p.f.u.), α approaches a saturation value, thus showing a proportional increase of tetrahedral and octahedral sheet lateral dimensions. Furthermore, α variation allows a coarse approximation of the threshold VIAl content, below which celadonitic substitution may not progress.

Geometric crystal chemical models for structural analysis of micas and their stacking polytypes

A sequence of progressively more realistic geometric crystal chemical models for TOT layers in mica is developed, starting from the usual main uniform distortions (octahedral flattening, tetrahedral rotation, octahedral counter-rotation) toward additional features as they are shown to be required. These additional features include tetrahedral basal flattening and apical bond adjustment, geometric meso-octahedral sheets (having unequal M1 and M2/M3 site bond lengths), and geometric heterooctahedral sheets (having unequal M1, M2, and M3 site bond lengths). A crystal chemical model for the unit cell of a 1M polytype with C2/m space group is developed from geometric homo-octahedral sheets (having equal M1, M2, and M3 site bond lengths) and is described using a minimal number of independent crystal chemical parameters: octahedral, tetrahedral basal, tetrahedral apical, and interlayer metal-anion bond lengths, and flattening angles of octahedral and tetrahedral sheets. The monoclinic lattice parameters (a, b, c, and β) and the tetrahedral rotation angle (α) follow from these assumed parameters. These models are designed to allow analyses (that are reported elsewhere) of both structural and lattice-parameter refinement data in terms of deviations from various predictions based on specified sets of crystal chemical assumptions. Fractional atomic coordinates are derived in terms of the atomic positions for the 1M unit cell of C2/m symmetry for each known homogeneous mica polytype with highest space group symmetry (polytype, space group = 2M 1 , C2/c; 2M 2 , C2/c; 2O, Ccmm; 3T, P3 1 12). These coordinates allow a structural analysis of diffraction data for different stacking polytype structures using the same 1M-type TOT layer as a modular unit.

Model pyroxenes II: Structural variation as a function of tetrahedral rotation

Model pyroxenes with regular tetrahedral and M1 octahedral coordination polyhedra have been derived. The M2 polyhedron is not constrained to be regular. These models are parameterized in terms of the O3-O3-O3 angle, θ, and the model O atom radius, r . Crystallographic parameters such as interatomic distances, unit cell volume, and packing distortion are determined as a function of the O3-O3-O3 angle. Results are compared with observed pyroxenes, providing insight into which interatomic interactions are important in determining pyroxene topology and behavior. Temperature is shown to favor polyhedral regularity in orthopyroxene and protopyroxene. Compression and expansion strain ellipsoids for observed and model pyroxenes are compared, demonstrating that a combination of tetrahedral rotation and isotropic compression approximately reproduces the compression ellipsoids of pyroxenes, but not the expansion ellipsoids.

A single-crystal study on the pressure behavior of phlogopite and petrological implications

The structural behavior of phlogopite was studied by in situ single-crystal X-ray diffraction (XRD) in a diamond-anvil cell, using crystals of composition [(K0.91Na0.02Ba0.03) (Fe2+ 0.65Fe3+ 0.163Al0.123Mg1.81 Ti0.149)Si2.708Al1.292O10OH1.725F0.175]. Lattice parameters were measured from 0.0001 to 6.5 GPa and fitted with a third-order Birch-Murnaghan equation of state (EoS). The resulting EoS parameters are: V = 497.1(1) A3, K = 54(2) GPa−1, and K ′ = 7(1); a = 5.336(1) A, K = 123(9) GPa, and K ′ = 3(2); b = 9.240(3) A, K = 128(15) GPa, and K ′ = 3(2); c = 10.237(6) A, K = 25(2) GPa, and K ′ = 5(1); the β angle increases linearly with pressure from 100.02(5) to 100.4(1)°. The structural evolution of phlogopite was studied by comparing five structural refinements performed with intensity data collected at 0.0001, 1.2, 3.2, 5.0, and 6.0 GPa. The interlayer site, where K is located, is about 4.5 times more compressible than the T-O-T unit. At the same time, the greater compression of the octahedral layer with respect to the tetrahedral one induces an increase in tetrahedral rotation angle α from 9.29 to 11.9°. Structural evolution with pressure yields a crystallographic rationale for the larger baric stability of K-deficient and Si-rich phlogopites, as observed from natural and experimental data. Combined high-pressure and thermal expansion data yield the approximate EoS: V = V (1 + 4.6 × 10−5 Δ T – 0.0167 Δ P ), where P is in GPa and T in °C. This equation was used to calculate phase equilibria in lherzolite compositions modeled in the simplified K2O-CaO-MgO-Al2O3-SiO2-H2O (KCMASH) system. The results show that the effect of phlogopite on bulk density is particularly significant at high pressure and temperature since, above the chlorite stability field, phlogopite is the only hydrous phase able to lower bulk density properties by about 0.5% with respect to the K-free bearing system.

The modulated crystal structure of antigorite: Them= 17 polysome

The modulated crystal structure of an antigorite polysome with m = 17 was refined by single crystal X-ray diffraction in the Pm space group, using highly ordered single crystals from Val Malenco, Italy. The chemical composition is (Mg 2.673 Fe 2+ 0.098 Fe 3+ 0.015 Al 0.035 Cr 0.007 Ni 0.003 Mn 0.002 ) Σ=0 . 823 (Si 1.997 Al 0.003 ) Σ=2O5 (OH) 3.639 . Lattice parameters [a = 43.505(6), b = 9.251(1), c = 7.263(1) Å, β = 91.32(1)°] were determined using a single-crystal diffractometer equipped with an area detector. The structure was refined using 9242 independent reflections, obtaining a final R4σ factor of 0.0577. A continuous, wavy octahedral sheet is linked to a tetrahedral sheet with tetrahedral apices alternatively pointing +c and -c. This sheet is located on the concave side of the octahedral-sheet wave. The octahedral sheet shows normal thickness for a serpentine of this composition, and does not have any internal offset. The tetrahedral sheet inverts its polarity through six- and eight-membered tetrahedral rings (6- and 8-reversals). Between reversals, 6-membered rings are distorted toward ditrigonal configuration, with tetrahedral rotation, α values, ranging along the wave from 4 to 13.6°. The two half-waves have curvature radii of 99.4 and 110.9 Å. Variable interlayer O-O distances occur, indicating the absence of homogeneous, continuous hydrogen bonding. The bond geometry, very similar to that of lizardite, suggests common crystal chemical and geochemical properties. The larger stability field of antigorite compared to lizardite is interpreted to arise from the occurrence of three-dimensionally connected chemical bonds.

Shirozulite, KMn2+3(Si3Al)O10(OH)2, a new manganese-dominant trioctahedral mica: Description and crystal structure

Shirozulite is a new Mn-dominant trioctahedral mica from the Taguchi mine, Aichi Prefecture, Japan. The mineral occurs in tephroite-rhodochrosite ores in contact with a Ba-bearing, K-feldspar vein. Shirozulite formed during regional low-P/T metamorphism and, thereafter, suffered thermal metamorphism from a local granodiorite. Grains of shirozulite are up to 0.5 mm across and have a typical micaceous habit. Color: dark reddish brown. Cleavage: (001), perfect. Optical properties: biaxial negative, 2V x = very small. Strongly pleochroic: X = pale yellow, Y = Z = pale brown, absorption X &lt; Y ≈ Z. Refractive indices: n α = 1.592(2), n β ≈ n γ = 1.635(2). The structural formula is (K 0.90 Ba 0.09 ) (Mn 2+ 1.53Mg 0.94 Fe 2+ 0.20 Ti 0.04 Al 0.29 ) (Si 2.54 Al 1.46 ) O 10 [(OH) 1.97 F 0.03 ], and the end-member composition is KMn 2+ 3 AlSi 3 O 10 (OH) 2 . Density: obs. = 3.20(3) g/cm 3 by pycnometer, calc. = 3.14(2) g/cm 3 . Shirozulite is monoclinic, C2/m, 1M polytype, a = 5.3791(7), b = 9.319(1), c = 10.2918(9) Å, β = 100.186(9)°, V = 507.8(1) Å 3 . The six strongest lines in the powder X-ray diffraction pattern are as follows: d (Å), l (%), (hkl): 10.16, 100, (001); 2.654, 96, (1̅31); 3.386, 51, (003); 1.556, 48, (3̅̅13); 2.467, 46, (1̅32); 2.202, 36, (1̅33). The crystal structure has been refined to an R value of 4.1% based on 663 observed reflections collected with MoKα X-radiation from a single crystal. The mean bond lengths, tetrahedral rotation, and octahedral flattening angles are as follows: &lt;T-O&gt; = 1.668, &lt;M1-O&gt; = 2.118, &lt;M2-O&gt; = 2.103, &lt;K-O&gt;(inner) = 2.995, and &lt;K-O&gt;(outer) = 3.376 Å, α = 8.36°, ψ M1 = 58.5°, ψ M2 = 58.2°. The apparent element distribution coefficient analyses among coexisting manganese or manganoan silicate minerals indicate that the trioctahedral mica structure cannot contain larger amounts of Mn 2+ relative to Mg and Fe 2+ than in olivine, pyroxenoid, and garnet.

Ab Initio Computational Crystallography of 2:1 Clay Minerals: 1. Pyrophyllite-1Tc

The interpretation of diffraction and spectroscopic data on clay minerals can be facilitated by first-principles quantum mechanical simulations based on density functional theory (DFT), which now has achieved sufficient accuracy to predict crystallographic properties of these minerals without recourse to empirical parametrization. To test the accuracy of DFT for simulating 2:1 clay minerals, we have performed calculations on a related layer aluminosilicate for which high-quality crystallographic data are available, pyrophyllite-1Tc. The computational scheme included a plane-wave basis set along with new pseudopotentials for Si, Al, and O generated using the Perdew−Wang 91 generalized gradient approximation for the core electron exchange-correlation functional, the same approximation that was used for the valence electrons. Our calculations allowed full structural relaxation and, therefore, were appropriate for comparison to ambient-pressure crystallographic data. The simulation results gave an excellent account of bond lengths and angles when compared to single-crystal X-ray diffraction data. Subtle features of the pyrophyllite structure, such as tetrahedral rotation and corrugation of the basal planes, also were captured accurately. The orientation angle of the structural OH groups in pyrophyllite was predicted to be 25.4° relative to the ab plane but was found to exist within a very broad minimum in respect to its effect on the total energy of the crystal. Overall we conclude from our results that high-quality DFT optimizations now can provide significant crystallographic information to aid in the interpretation of diffraction and spectroscopic data on layer type aluminosilicates.

The Crystal Structure of Roscoelite-1M

AbstractSingle-crystal X-ray diffraction experiments were carried out on roscoelite crystals from Reppia, Val Graveglia, Italy. Roscoelite [structural formula: XII(Ba0.006K0.994)IV(Si3.150Al0.850) VI(Al0.040Fe0.150Mg0.100Mn0.062V1.696Ti0.003)O10(OH)2] shows a near-perfect three-dimensional stacking order with cell parameters a = 5.292(1), b = 9.131(2), c = 10.206(3) Å, β = 100.98(2)° and space group C2/m, which indicate a 1M polytype. The crystal structure was refined on the basis of Fo2 for 846 unique reflections to R1 = 3.29% calculated using 746 unique observed reflections [|Fo| ⩾ 4σ(Fo)]. The mean tetrahedral cation–oxygen atom distance, &lt;T−O&gt; = 1.641 Å, is close to the mean &lt;T−O&gt; value obtained for dioctahedral true micas from the literature, whereas the octahedral sheet is characterized by a larger cis-octahedral cation–oxygen atom bond distance &lt;M2−O&gt; = 2.020 Å which, together with the mean electron count, is consistent with V occupancy. The presence of V within the octahedral sheet produces the smallest tetrahedral rotation (α = 2.3°), the lowest flattening of the basal oxygen surface (Δz = 0.118 Å) and the narrowest interlayer separation (3.030 Å) in dioctahedral micas.

Pressure-induced phase transition in synthetic trioctahedral Rb-mica

The crystal structure of a synthetic Rb analog of tetra-ferri-annite (Rb–TFA) 1M with the composition Rb0.99Fe2+3.03(Fe3+1.04 Si2.96)O10.0(OH)2.0 was determined by the single-crystal X-ray diffraction method. The structure is homooctahedral (space group C2/m) with M1 and M2 occupied by divalent iron. Its unit cell is larger than that of the common potassium trioctahedral mica, and similar lateral dimensions of the tetrahedral and octahedral sheets allow a small tetrahedral rotation angle α=2.23(6)°. Structure refinements at 0.0001, 1.76, 2.81, 4.75, and 7.2 GPa indicate that in some respects the Rb–TFA behaves like all other micas when pressure increases: the octahedra are more compressible than the tetrahedra and the interlayer is four times more compressible than the 2:1 layer. However, there is a peculiar behavior of the tetrahedral rotation angle α: at lower pressures (0.0001, 1.76, 2.81 GPa), it has positive values that increase with pressure [from 2.23(6)° to 6.3(4)°] as in other micas, but negative values −7.5(5)° and −8.5(9)° appear at higher pressures, 4.75 and 7.2 GPa, respectively. This structural evidence, together with electrostatic energy calculations, shows that Rb–TFA has a Franzini A-type 2:1 layer up to at least 2.81 GPa that at higher pressure yields to a Franzini B-type layer, as shown by the refinements at 4.75 and 7.2 GPa. The inversion of the α angle is interpreted as a consequence of an isosymmetric displacive phase transition from A-type to B-type structure between 2.81 and 4.75 GPa. The compressibility of the Rb–TFA was also investigated by single-crystal X-ray diffraction up to a maximum pressure of 10 GPa. The lattice parameters reveal a sharp discontinuity between 3.36 and 3.84 GPa, which was associated with the phase transition from Franzini-A to Franzini-B structure.